The present invention relates to a monolithic resonator for use in a vibrating beam accelerometer (VBA) in which the proof mass, the proof mass suspension system and the resonator are made from a single piece of quartz crystal. The sensing element or resonator is a force-sensitive, vibrating beam. The vibrations of the vibrating beam are sustained by the combined interaction of the electrode pattern plated on the beam, the piezoelectric properties of the quartz crystal material and the electrical energy supplied by the electronic oscillator circuit. When the vibrating beam experiences axial tension, its vibration frequency increases and inversely, when the beam experiences compression, its vibration frequency decreases. The advantages of such a sensing element in an accelerometer application include a direct digital output as well as good stability and low temperature sensitivity due to the properties of quartz crystal. The advantages to be gained by fabricating the structure from a single piece of quartz crystal include low cost, elimination of many assembly operations, elimination of joints, and compact size.
Single piece VBA's, per se, are known in the art, for example, see U.S. Pat. No. 4,804,875 which discloses a monolithic piezoelectric structure in both single and dual vibrating beam configurations. See also, as representative examples, U.S. Pat. Nos. 4,658,174; 4,658,175; 4,656,383; 4,446,394; 4,445,065, 3,479,536 and 3,470,400.
In one exemplary embodiment in accordance with this invention, the sensing element comprises a vibrating beam, isolator masses and isolator beams generally of the type shown in U.S. Pat. No. 4,743,790, owned by the assignee of this invention. In accordance with this invention, however, one end of the resonator or sensing element is attached to the mount structure, while the other end is attached to the proof mass structure, and these two structures are joined together by means of a series of flexure beams. Absent the vibrating beam sensing element, the flexure beams would otherwise permit the mount structure and the proof mass structure to move relative to each other in a parallel like motion along a given axis.
The mount structure and proof mass structure each include appendages, for mounting the sensing element to an external surface such as an accelerometer housing, and for mounting an additional proof mass to increase the total mass of the device, respectively.
As will be explained in greater detail below, the present invention also incorporates a non-planar design which has many advantages over prior art constructions. For example, typical prior art designs utilize planar construction whereby the thickness of all parts of the structure are equal. The accelerometer design of this invention utilizes a non-planar approach whereby the outer structure, consisting of the flexure beams and mount structures, may be from three to ten times thicker than the inner structure vibrating beam portion. This arrangement achieves a structure which can withstand Z axis acceleration and vibration, while being sufficiently sensitive to X input axis acceleration. In addition, structure stiffness along the Z axis is achieved by making the outer structure thickness as great as necessary. If the thickness of the inner structure is increased correspondingly with the thickness of the outer structure, the force sensitivity of the vibrating beam would decrease since the sensitivity of the vibrating beam sensing element is inversely proportional to the Z axis thickness. Low vibrating beam sensitivity would mean that a heavier proof mass would be needed which in turn, would place the outer structure under a greater load to Z axis acceleration. Thus, it has been determined that a single thickness structure is not practical because the outer structure Z axis stiffness requirements cannot be conveniently made compatible with the vibrating beam inner structure sensitivity requirements. By adopting a non-planar approach, the outer and inner structure stiffness requirements are no longer related, so that the Z axis stiffness requirements and vibrating beam sensitivity requirements can be conveniently met.
Another advantage of the present invention is the utilization of multiple flexure beams. This arrangement results in an outer structure which is very stiff to Z axis loads but very compliant to X axis loads, as will be explained in greater detail below.
feature of the present invention is the placement of all flexure beams at the bottom end of the structure. This keeps the center of gravity of the proof mass low on the structure.
Another feature of the present invention relates to the addition of the above described mount appendages which enables the structure to be attached to an external case or housing, and also enables attachment of an additional proof mass. In this regard, if the mount structure portion were used to mount the sensing element directly to an external case housing, the mount structure would experience strains due to clamping forces, differential thermal expansion effects, etc. These strains would distort the structure and the vibrating beam would experience erroneous axial forces. Locating the clamping or other mounting means on the mount structure appendage does not cause such errors because the appendage is not located in a region where such strains would cause vibrating beam error forces. The same is true with respect to the attachment of an additional proof mass to the proof mass appendage.
It will be appreciated that various alternatives to the above described configuration are within the scope of the invention. For example, while in one presently preferred embodiment three flexure beams are utilized, it will be understood that any number of flexure beams greater than two may be utilized. Specifically, the number and geometry of the flexure beams depends on the ratio of X axis compliance to Z axis stiffness required, and also on anticipated stress levels.
It will also be understood that the use of the above described appendages may be reversed, i.e., the external mount appendage can be used for mounting the proof mass and the proof mass appendage can be used for the external mounting.
In the above described embodiment, the proof mass structure extends only to the lower end of the inner structure, whereas the mounting structure extends the full height of the device. In another exemplary embodiment of the invention, however, the proof mass structure may be extended to the full height of the device, and one or more flexure beams added to the top part of the structure. As a result of this configuration, the rather fragile inner structure is further protected during handling by being more fully surrounded by the relatively thick and rugged outer structure. In addition, the added flexure beam or beams at the top of the structure may provide more overall stiffness for the entire structure.
In another exemplary embodiment, only one mounting structure appendage is required since sufficient proof mass is provided by the resonator structure per se. In this embodiment, a pair of vertically spaced flexure beams are provided in the lower portion of the structure.
In its broader aspects, the present invention provides a monolithic resonator for a vibrating beam accelerometer which comprises an outer structure including a mounting structure, a proof mass structure, a plurality of flexure beams extending between the mounting and proof mass structures; and an inner structure including first and second isolator masses, first and second isolator beams connected to one portion of the isolator masses, respectively, and a vibrating beam extending between other portions of the isolator masses; wherein the outer structure has a thickness greater than said inner structure.
Objects and advantages of this invention in addition to those noted above will become apparent from the detailed description which follows.